The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 3,379,473; 5,839,781; 6,120,094; 6,213,551; 6,244,658; 6,402,242 and 6,997,510; and
U.S. Published Patent Application Nos.: 2007/0096522 and 2011/0148161.